Aqueous foams are employed during wide bore tunnel excavation through clayey soils using tunnel boring machines (TBMs). The foams are used as a means to add fluids to fill the drilling chamber to help maintain pressure without adding too much water. Further, they help prevent the clay from clogging up the cutter head of the machine and also the conveyor belts taking away the excavated spoil. The mechanism of how the foam works is not well understood but it appears that the foam prevents clay lumps from contacting one another, dilutes the day and prevents lumps from hydrating to the point at which the clay becomes sticky. It is also thought that the foam acts as a lubricant.
One problem with the surfactant-stabilised foams is that on contact with the clay, the surfactant and water associated with the foam are attracted to and adsorbed by the high surface area day and the result is a much shortened life for the foam due to collapsing. This leads to large amounts of foam being required in order to replace collapsed foam, causing problems with evacuation of liquids from the bore hole, the requirement for fast delivery of foam to the bore hole, and for supplying and maintaining large amounts of foam composition at the drilling site. Furthermore, after downtime of the tunnel boring machine, it is often found that the foam has broken down upon start-up of the TBM.
WO 01/12952 A1 discloses a foamed aqueous solution for use in tunnel excavation, containing an anionic surfactant and a β-naphthalene sulphonate-formaldehyde condensate (BNS). Very large amounts of surfactant solution are required in this case, presumably due to the lack of stability of the foamed solution.
The state of the art therefore constitutes a problem.